1. Field of the Invention
The present invention relates to an optical disk apparatus for reading data recorded on optical disks such as CDs and DVDs and recording data on the optical disks.
2. Description of the Related Art
An optical disk apparatus have so far found a wide application in reading the data recorded on optical disks such as CDs and DVDs and recording the data on the optical disks. The optical disk apparatus radiates a laser beam to an optical disk set on the main body to read recorded data or record the data. The optical disk apparatus conduct a focus control in which a focusing position of a laser beam radiating to an optical disk is adjusted to a recording plane of the optical disk at the time of reading or recording data, and a tracking control in which a position of radiating a laser beam is adjusted to the center of the track of the optical disk. The focus control is a control in which an objective lens arrayed between the optical disk and LD (light source) is moved toward the direction of contacting on and off an optical disk. Further, the tracking control is a control in which the objective lens and the main body of a pickup head are moved radially toward the optical disk. The pickup head is provided with an LD, an objective lens and a biaxial actuator which allows the objective lens to move in the direction of contacting on and off the optical disk and radially toward the optical disk. Further, the pickup head is attached radially toward the optical disk in a freely movable way. A thread motor allows the pickup head to move radially toward the optical disk. The focus control is conducted in accordance with a focus error signal (hereinafter referred to as FE signal) which indicates a deviation extent from a focusing position of a laser beam giving radiation to the recording plane of the optical disk, and the tracking control is conducted in accordance with a tracking error signal (hereinafter referred to as TE signal) which indicates a deviation extent of the center of the track of the optical disk from the position of radiating a laser beam.
In addition, since DVDs record data at a higher density than CDs, the track pitch and the pit size of DVDs are smaller than those of CDs. BDs (Blu-ray disk) are now under development, which can record data at a higher density than DVDs. In order to record or reproduce data on optical disks such as DVDs and BDs at a higher density, it is necessary to accurately control a spot diameter of a laser beam focusing light on the recording plane of the optical disk. Unless an optical axis of a laser beam radiating from the pickup head is vertical to the recording plane of the optical disk, coma aberration will occur in the laser beam concentrated on the recording plane of the optical disk. Therefore, in order to control more accurately the spot diameter of a laser beam concentrated on the recording plane of the optical disk, it is necessary to control the optical axis of the laser beam radiating from the pickup head vertical in relation to the recording plane of the optical disk. On the other hand, since the optical disk has a certain warp, provided is a mechanism for controlling an angle (tilt angle) for tilting the pickup head or the objective lens according to the warp of the optical disk (hereinafter referred to as tilt control mechanism). An optical disk apparatus equipped with a tilt control mechanism for tilting a pickup head has been disclosed in JP-A-2001-195763 and JP-A-5-63332, for example. Further, an optical disk apparatus equipped with a tilt control mechanism for tilting an objective lens has been disclosed in JP-A-2004-118909, for example.